Exploit Mitigation Technologies
OpenBSD's exploit mitigation technologies are designed to kill applications that behave "inappropriately".
Stackgap
Propolice
Non-exec stack
W^X
randomized library allocation
randomized mmap()/malloc()
StackGhost
Most of this is enabled by default with little or no performance impact
See Theo de Raadt's "Exploit Mitigation Techniques" for the grusome details